Photolithography :

In different processing steps such as oxidation, etching, metal and
polysilicon deposition and ion implantation it is required to mark the
certain area on the chip by using an optical mask because of such masking
the desired processing step is applied to the selected areas on the chip.
The technique to do this selective masking is called as photolithography.
In the photolithography, first â€œreticleâ€ which is a transparent silica
plate containing the pattern is generated. The reticle typically contains
the patterns corresponding to a single chip or die rather than the entire
wafer. When the pattern is generated for the entire wafer then transparent
silica plate is named as â€œmaskâ€ instead of â€œreticleâ€.

The reticle or mask is usually created by a computer controlled electron
beam driven by the circuit layout data using pattern generator software.
The different steps involved in the photography include :

Step 1 : Oxidation layering :

In this step a thin layer of SiO2 is deposited over the complete
wafer by exposing it to oxygen.

Step 2 : Photoresist coating :

In this step a light sensitive polymer is coated on the wafer. This
material is soluble in organic solvent but has the property that when
exposed to light the light exposed regions becomes insoluble in organic
solvent this types of photoresist is called as negative photoresist. A
positive photoresist has the opposite properties i.e. it originally
insoluble but when exposed to light it becomes soluble in organic solvent.

Step 3 : Stepper exposure :

In this step a reticle or (a mask) which contains patterned layout is
brought close proximity to the wafer. The mask is transparent in some
regions and opaque in the regions which we wanted to process. The
combination of mask and wafer is now exposed to ultraviolet light. If the
negative photoresist is coated on the wafer then the region where the mask
is transparent, the photoresist becomes insoluble.

Step 4 : Soft baking :

Now the wafers are kept in either on acidic or basic solutions to remove
the non exposed areas of photoresist. Once the exposed photoresist is
removed the wafer â€œsoft bakedâ€ at a low temperature to harden the remaining
photoresist.

Step 5 : Acid etching :

The material from the selected areas which are not covered with photoresist
is removed by using different types of acid base and acoustic solutions.
Because of the dangerous nature of these solvents, safety and environmental
impact is primarily concerned in this step.

Step 6 : SRD :

This is called as Spin, Rinse and Dry step which cleans the wafer with
deionised water and dries it with Nitrogen. In integrated circuits the
smallest particle of dust or dirt can destroy the circuitry, in order to
prevent this the processing steps are performed in ultra clean rooms where
the number of particles per cubic foot of air ranges between 1 and 10 also
called as
â€œ Class 1â€ or â€œClass 10â€ clean rooms. Because of such fabrication
requirements the cost of integrated circuit fabrication is very high.

Step 7 : Various process steps :

Now the exposed area can be used for other processing steps such as ion
implantation, plasma etching or metal deposition discussed in subsequent
sections.

Step 8 : Ashing :

This step is used to remove the remaining photoresist without damaging the
other wafer areas where devices are formed. As this photoresist was exposed
to light this is hard and insoluble in solvent hence to remove this, high
temperature plasma is used. Figure below shows the various steps involved
in the photolithography process.

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